The present invention relates to an electrode metal material for electrical components such as capacitors and batteries which are used in contact with electrolyte, to a capacitor and a battery formed of the electrode metal material, and to a method of producing the electrode metal material and the capacitor and battery thereof.
At present, there are, for example, electric double-layer capacitors and electrolytic capacitors available as electrical components which are used in contact with electrolyte. Such electric double-layer capacitors have been applied to large-capacitance capacitors chargeable at up to about 3 V, and used for backup power sources of microcomputers, memory devices, timers, and the like.
Typically, an electric double-layer capacitor comprises a pair of polarizable electrodes or double-layer electrodes disposed face-to-face via an insulating separator therebetween and immersed in electrolyte. The electrode is produced by applying an activated carbon layer on the surface of an electrode metal material made of a valve metal and used as a mechanical supporter and, at the same time, electric collector.
Some types of electric double-layer capacitors use an organic-solvent based electrolytic solution as electrolyte, such as a tetraethyl ammonium salt which is added to an organic solvent, such as propylene carbonate. The examples of conventional electric double-layer capacitors using organic-solvent based electrolyte include a type in which a pair of electric double-layer electrodes is wound and enclosed in a container, and another type in which a pair of double-layer electrodes is laminated or stacked, both types having been disclosed in U.S. Pat. No. 5,150,283.
In the case of the winding type of capacitors, as shown in FIG. 7, an electrode metal material 1 is formed of etched aluminum foil having a thickness of 20 to 50 xcexcm, and a paste obtained from a powder mixture of activated carbon particles, a desired binder and a desired conductive agent is applied to the above-mentioned metal foil to form a film. This film, that is, an activated carbon layer 30 (a polarizable electrode) mainly consisting of activated carbon particles, is used to form an electric double-layer electrode 3.
A lead 6 is connected to each of the electrode metal materials 1 of the pair of electric double-layer electrodes 3 and 3, respectively. These electrodes 3 and 3 are disposed face-to-face with a separator 5 therebetween and wound like a coil. The electric double-layer electrodes is immersed in non-aqueous electrolyte under vacuum to impregnate the activated carbon layers 30 and the separators 5 with the electrolyte, then placed in an aluminum case 70, the opening 7 of the aluminum case 70 being sealed with a watertight packing 8. The electrolyte in the electric double-layer capacitor has used polypropylene carbonate as an organic solvent, and a tetraethyl ammonium salt as an electrolyte, for example.
Furthermore, in a button-type electric double-layer capacitor, schematically shown in FIGS. 9 and 10, activated carbon layers 30 are joined to disc-like sheets 1 made of a valve metal material, respectively, to form a pair of double-layer electrodes 3. The pair of double-layer electrodes 3 and 3 are disposed face-to-face via an insulating separator 5 therebetween, and accommodated in a metal container comprising two mating members. The valve metal material sheets of the two double-layer electrodes are joined to the inner surface sides of the bottom member 60 and the lid member 61 of the metal container. Both the bottom and lid members are joined to each other so as to be watertight by using an insulating ring packing 69 at the peripheral portion thereof. The interior of the capacitor is filled with non-aqueous electrolyte so that the double-layer electrodes and the activated carbon layers are immersed therein sufficiently. The non-aqueous electrolyte is a solution of tetraethyl ammonium perchlorate added in propylene carbonate in the same way as described above.
An electrolytic capacitor is known as a capacitor in which non-aqueous electrolyte is used. In the anode of the capacitor, a dielectric film is formed by chemically treating the valve metal foil. In the cathode, the valve metal foil is used as it is. Usually, both the electrodes are disposed face-to-face, wound into a coil, and hermetically enclosed in a container while being immersed in electrolyte.
In the case of the conventional electric double-layer capacitor, the valve metal sheet or foil, on which a polarizable electrode is formed as a film, has a naturally oxidized film specific to the valve metal constituting an electrode structure while the foil is handled. When this foil is used to form an electrode structure, a thin, insulating oxidized film 4 is frequently formed at the interface between the aluminum foil 1 used as a valve metal material and the polarizable electrode 3, as schematically shown in FIG. 6.
Furthermore, the above-mentioned non-aqueous electrolyte typically includes slight amounts of water and oxygen. For this reason, the valve metal material constituting the electrode structure reacts with the water content in the electrolyte during use of the capacitor, and the surface of the metal is oxidized. Therefore, when the electric double-layer capacitor formed of this kind of metal is used for extended periods of time, its equivalent series resistance (ESR), i.e., the internal resistance of the capacitor used as a power source, increases gradually, and, in some cases, its capacitance decreases.
This problem due to the oxidation of the metal portion of the electrode has also occurred in the case of the button-type electric double-layer capacitor in the same way.
Furthermore, the anode of the electrolytic capacitor using non-aqueous electrolyte is provided with a dielectric insulating layer formed by anodizing a valve metal such as aluminum. In addition, its cathode in direct contact with the electrolyte is also formed of the valve metal such as aluminum. In this case, an oxide film is formed on the surface of the metal used for the cathode because of oxidation with the water content in the electrolyte. This causes a problem of the capacitor increasing in internal resistance, just like the problem described above.
With respect to batteries using electrodes in contact to non-aqueous electrolyte, a lithium ion secondary battery is known which has high charge-discharge cycle performance with high energy density in a compact shape.
A lithium ion secondary battery, as shown in FIG. 11, comprises a positive electrode 35, a negative electrode 37, facing to the positive electrode, a film separator 5 for separating both electrodes 35 and 37, and a non-aqueous electrolytic solution in which both the electrodes are placed and contained in a casing 71. The positive electrode 35 is, as an example, formed of a mixture of positive active substance such as LiCoO2, conductive material such as acetylene black, and a binder including carboxylmethylcellulose and polyflorovinylidene which mixture is applied on both sides of aluminum foil as an electrode metal material 1 for an electric collector. On the other hand, the negative electrode 37 is formed of a mixture of negative active substance such as graphite and a binder such as carboxylmethylcellulose and styrene-butadiene rubber which mixture is applied on both sides of copper foil as an electric collector. The electrolytic solution is a non-aqueous solvent of a mixture of propylenecarbonate and 1,2-dimethoxyethane containing LiPF6 as electrolyte. A porous polypropylene film is used as a separator.
In conventional lithium ion secondary batteries, aluminum foil is formed with natural oxide film on its surface during dealing with the foil so that thin isolating film have often been formed in the interface between the aluminum foil and the positive electrode on the aluminum foil.
Further, since the above non-aqueous electrolytic solution also contains slight amount of water and oxygen, the aluminum foil in the battery have been oxidized on its surface, in use, gradually over long time due to reaction of aluminum surface with water in the electrolytic solution, causing a lithium ion secondary battery to increase in equivalent series resistance, i.e., internal resistance and resulting in low capacity at high discharge rate.
Accordingly, an object of the present invention is provide a valve metal material capable of being formed into electrodes used in contact with non-aqueous electrolyte to reduce internal resistance of a capacitor or battery.
Another object of the present invention is to provide a method of producing a valve metal material capable of being formed into electrodes used in contact with non-aqueous electrolyte to reduce internal resistance of such a capacitor or battery.
A still another object of the present invention is to provide a capacitor capable of having a low internal resistance by restricting the change in the resistance of the electrode metal material constituting the electrodes used in contact with non-aqueous electrolyte.
A still another object of the present invention is to provide a non-aqueous secondary battery having low internal resistance by restricting the change in the resistance of the electrode metal material constituting the electrodes used in contact with non-aqueous electrolyte.
A yet still another object of the present invention is to provide a method of producing a capacitor capable of having a low internal resistance by restricting the change in the resistance of the electrode metal material constituting the electrodes used in contact with non-aqueous electrolyte.
A yet still another object of the present invention is to provide a method of producing a non-aqueous secondary battery having low internal resistance by restricting the change in the resistance of the electrode metal material constituting the electrodes used in contact with non-aqueous electrolyte.
An electrode metal material in accordance with the present invention is formed of a valve metal material containing carbon particles on the surface, and is used to form electrodes. The carbon particles in the carbon-containing metal material ensure direct contact with a conductor (including electrolyte) to electrically connect the electrode metal material to the conductor.
In particular, the carbon-containing metal material comprises a valve metal material and numerous carbon particles fixed in the surface of the valve metal material and exposed to the surface. In the present invention, the carbon particles may be projected slightly so as to be exposed to the surface of the valve metal material in order to enhance the conductivity and joining characteristic to a conductor to become contact therewith.
The electrode metal material in accordance with the present invention may be used to obtain electrode structures used in contact with non-aqueous electrolyte. This kind of carbon-containing metal material itself may be an electrode making contact with electrolyte. Alternatively, the electrode metal material may have an activated carbon layer coated on the surface, i.e., a polarizable electrode. The former corresponds to the cathode of an electrolytic capacitor, and the latter corresponds to the double-layer electrode of an electric double-layer capacitor.
Further, the electrode metal material may be used to support a positive electrode including a positive active substance on the surface of the electrode metal material, the positive electrode being used for a non-aqueous electrolytic secondary battery, e.g., a lithium ion secondary battery.
In the electrolytic capacitor, the carbon particles of the carbon-containing metal material, exposed to the surface thereof, can make direct contact with the electrolyte to ensure conductivity between the metal material and the electrolyte. In addition, inside the electric double-layer capacitor, the carbon particles of the carbon-containing metal material, exposed to the surface thereof, can make direct contact with the activated carbon layer to ensure conductivity between the metal material and the activated carbon layer. Further, in the lithium ion secondary battery, the carbon particles of the carbon-containing metal material are exposed to the surface thereof, to make direct contact with the active substances in the positive electrode, ensuring conductivity between the electrode metal material and the positive electrode.
In any of the cases, even if the carbon-containing metal material makes contact with electrolyte solution, and the metallic surface thereof is oxidized by water contained in non-aqueous electrolyte, the conductivity noted above remains almost unchanged over long time periods.
More particularly, numerous carbon particles may project on the surface of the valve metal material. Therefore, it is preferable that only the metal surface of the valve metal material may be removed such that the carbon particles are left projected on the removed surface. This projection configuration of the surface of the valve metal material ensures conductivity to the activated carbon layer in the capacitor or active substance in the battery, and also enhances strength of joining to the activated carbon layer or positive electrode.
More particularly, the metallic surface of the valve metal material may be coated with a passive film. In this case although the metallic surface of the valve metal material itself may lose conductivity, the metallic surface is prevented stably from oxidation because of no contact with the electrolyte, and the valve material has stable conductivity via the carbon particles for extended periods of time.
The valve metal material in accordance with the present invention can be formed into sheet. The term xe2x80x9csheetxe2x80x9d herein refers to plate, sheet, film and foil. The valve metal material may be formed of products other than sheet, having a small thickness with a desired shape. The electrode metal material may have a shape of net or punched plate. this may be is adequate to apply, for example, the positive electrode thereon to produce non-aqueous secondary battery.
The sheet and other formed products may include carbon particles at least on one side thereof and also may include carbon particles on both sides thereof.
A method of producing a valve metal material for electrodes in accordance with the present invention contains driving or squeezing numerous carbon particles into the surface thereof. Pressing using dies or rolling using rollers may be employed to drive powder of carbon particles into a valve metal sheet, then, carbon particles being fixed in the surface of the valve metal sheet with the particle exposed on the surface.
Another method may be adopted where a slurry of carbon particles is applied and dispersed on a surface of a valve metal material and pressed to squeeze the carbon particles into the surface, then, obtaining carbon-containing electrode metal material. The carbon slurry may comprise carbon particles and a solvent, particularly, volatile dispersing liquid without any binder used. The dispersing liquid may be water, alcohol or other volatile liquid because after drying, it is preferable that only carbon particles remain dispersed on the surface without containing any impurity such as binder solid.
Prior to pressing in the above methods, the valve metal material may preferably be roughened on the surface, particularly be made porous in a thin layer of the surface, facilitating carbon particles to engage and embed in the porous surface layer effectively.
Also, a method of producing the valve metal material for electrodes in accordance with the present invention may include a step wherein the powder material for the valve metal and carbon particles are semi-melted in a mixture condition and subjected to pressure so as to be formed into a dense metal ingot. The metal ingot, including carbon particles dispersed inside, is forged or rolled into a product having a desired shape, and then the carbon particles are exposed to the surface of the product.